Waste heat may be rejected to the atmosphere by dry or sensible heat exchangers. In a dry or sensible heat exchanger, there are two fluids: an air stream and a process fluid stream. In a closed system, the process fluid stream is enclosed so that there is no direct contact between the air stream and the process fluid stream; the process fluid stream is not open to the atmosphere. The enclosing structure may be a coil of tubes. Sensible heat is exchanged as the air stream is passed over the structure enclosing the process fluid stream. In the art these structures are known as “compact heat exchangers.”
In most climates, evaporative heat exchangers offer significant process efficiency improvements over dry heat exchangers. One type of evaporative heat exchanger is a direct evaporative heat exchanger, also known in the industry as an open cooling tower. In a direct heat exchanger, only an air stream and an evaporative liquid stream are involved; the evaporative liquid stream is usually water, and the two streams come into direct contact with each other.
Another type of evaporative heat exchanger is an indirect closed circuit evaporative heat exchanger, where three fluid streams are involved: an air stream, an evaporative liquid stream, and an enclosed process fluid stream. The enclosed fluid stream first exchanges sensible heat with the evaporative liquid through indirect heat transfer, since it does not directly contact the evaporative liquid and then the air stream and the evaporative liquid exchange heat and mass when they contact each other.
Another type of evaporative heat exchanger is a combined direct and indirect closed circuit evaporative heat exchanger. Examples of combined systems are disclosed in U.S. Pat. No. 5,435,382, U.S. Pat. No. 5,816,318 and U.S. Pat. No. 6,142,219.
Both dry and evaporative heat exchangers are commonly used to reject heat as coolers or condensers. Evaporative coolers reject heat at temperatures approaching the lower ambient wet bulb temperatures, while dry coolers are limited to approaching the higher ambient dry bulb temperatures. In many climates the ambient wet bulb temperature is often 20° to 30° F. below the ambient design dry bulb temperature. Thus, in an evaporative cooler, the evaporative liquid stream may reach a temperature significantly lower than the ambient dry bulb temperature, offering the opportunity to increase the efficiency of the cooling process and to lower the overall process energy requirements. Evaporative condensers offer similar possibilities for increased efficiency and lower energy requirements. In spite of these opportunities to increase process efficiencies and lower overall process energy requirements, evaporative cooling and evaporative condensing are often not used due to concern about water consumption from evaporation of the evaporative liquid and freezing potentials during cold weather operation.
In addition, both sensible and evaporative heat exchangers are typically sized to perform their required heat rejection duty at times of greatest thermal difficulty. This design condition is typically expressed as the summer design wet bulb or dry bulb temperature. While it is often critical that the heat rejection equipment be able to reject the required amount of heat at these design conditions, the duration of these elevated atmospheric temperatures may account for as little as 1% of the hours of operation of the equipment. The remainder of the time, the equipment may have more capacity than required, resulting in unnecessary usage of additional evaporative liquid.
U.S. Pat. No. 6,142,219 discloses a closed circuit heat exchanger having three heat exchange sections: a dry indirect contact heat exchange section; a second indirect contact heat exchange section that is operable in either a wet or dry mode; and a direct contact heat exchange section. As a fluid cooler, a connecting flow path connects the dry indirect contact heat exchange section to the second indirect contact heat exchange section. A bypass flow path extends from the dry indirect contact heat exchange section to the process fluid outlet. A modulating valve is at the outlet so that process fluid can be selectively drawn from the dry indirect contact heat exchange section alone, from the second indirect contact heat exchange section in series with the dry indirect contact heat exchange section, or from both the dry and second indirect contact heat exchange sections and mixed. Separate air streams pass through the second indirect and direct contact heat exchange sections before entering the dry indirect contact heat exchange section. As a condenser, process fluid is directed to the dry indirect contact heat exchange section alone or to the dry and second indirect contact heat exchange sections in parallel by valves in the process fluid supply lines. In another embodiment, the process fluid flows in series from the dry to the second indirect contact heat exchange section. The system is operable in different modes to extract heat from the process fluid in the most efficient way with respect to annual water consumption. At low temperatures, the system operates dry with primary heat extraction performed by the dry indirect contact heat exchange section. At higher temperatures, the air streams may be adiabatically saturated with evaporative liquid to pre-cool them below the dry bulb temperature before entering the dry indirect contact heat exchange section. At still higher temperatures, the apparatus may be operated in a wet mode with the primary heat extraction performed by the second indirect contact heat exchange section. Heat is extracted from the process fluid while selectively distributing or not distributing the evaporative liquid over the second indirect contact heat exchange section.